Two aspects of "boundary" lubrication by synovial fluid are being studied in an in vitro system: (1) the nature of surfaces which can be lubricated by synovial fluid; (2) the lubricating ability of synovial fluid in various physiological and disease states. Lubrication will be compared with the dynamic viscosity of the fluid using a miniaturized Couette viscometer (McCutchen). The apparatus for measuring both the friction and viscosity is suitable for the small quantities of synovial fluid available from clinical subjects. The findings will be relevant to the pathogenesis of cartilage abrasion in primary and secondary types of osteoarthritis.